We prospectively evaluated lower respiratory secretions obtained by bronchoalveolar lavage and stained with Oil-Red-O in patients with a variety of parenchymal diseases and in normal subjects. Our subjects included aspirators (n = 9), nonaspirators (n = 40), and normal subjects (n = 9). By grading the amount of intracellular Oil-Red-O per 100 alveolar macrophages, we computed a semiquantitative, lipid-laden alveolar macrophage index. The mean index for aspirators of 207 +/- 80 was significantly greater than the mean indexes of nonaspirators (121 +/- 97, p less than 0.02) and normal subjects (0.6 +/- 1.7, p less than 0.001). An index of greater than or equal to 100 had a sensitivity and negative predictive value of 100% each, a false negative rate of 0%, and a specificity of 57%. We conclude that (1) the mere presence of lipid-laden alveolar macrophages in lower respiratory secretions is a nonspecific marker of parenchymal lung disease; (2) the computation of a lipid-laden alveolar macrophage index may be helpful in excluding aspiration as a cause of parenchymal lung disease.